Digital wireless communication systems, such as Code Division Multiple Access (CDMA) cellular systems, are increasingly utilizing packet based transmission protocols to transport information between infrastructure entities. These protocols include frame relay and Asynchronous Transport Mechanism (ATM). The information to be transported includes encoded voice packets, data and control information.
In a digital cellular communication system, a mobile unit communicates over the air interface with a Base Station (BS), which in turn is connected to a Base Station Controller (BSC). Several physical communication mediums can be used between the BS and the BSC, including T1 span lines, microwave links and satellite links. These transport mediums can support several transport mechanisms. The transport mechanisms are typically grouped into two categories, circuit based and packet based. Circuit based protocols typically partition the medium into fixed bandwidth channels in a Time Division Multiplex (TDM) configuration. Each channel is dedicated to one phone call and tends to be very synchronous in operation.
Packet based protocols typically share the medium on a as-needed basis. Greater utilisation efficiency can be achieved by sharing the bandwidth, resulting in the support of more channels. In the case of speech information, conventional code division multiple access (CDMA) cellular systems take advantage of the statistical nature of speech to increase the number of channels handled by a given data "pipe". This is particularly the case with CDMA systems where speech coding algorithms in voice coders (vocoders) generate variable speech packet sizes that are directly related to the amount of speech activity at a given time. With the increased usage of the internet, data services is also increasingly showing a highly variable data rate.
Although packet based mechanisms will typically support a greater number of channels, they suffer from a large variance in transmission delay. This is due to the asynchronous nature of the packet transmission. As packets arrive for transmission on the data pipe, they have to be queued until space can be found to send them. The amount of time that they have to be queued depends heavily on the number of, and size of, other packets that are currently pending. In a cellular system, components at each end of the BS to BSC packet pipe rely on a synchronous data flow. These include the air interface between the BS and the mobile units as well as the interface to and from the vocoders. To support these synchronous interfaces an asynchronous to synchronous conversion is required. Typically this is handled by allowing for the worst case packet variance and buffering data appropriately.
One of the primary benefits of current CDMA systems is the ability to perform soft handoffs. With soft handoff each mobile unit can be in simultaneous communication with multiple BSs. For example if a mobile unit is in a 2-way soft handoff call, framed information will be sent to two BSs, the BSs will process the information, packetize it (in the case of a packet based network) and send it to the BSC. A selector function will analyse the packets received from the two BSs and select the best one for forwarding to , in the case of encoded speech, the vocoder. In the other direction, a speech encoded packet from the vocoder will be duplicated and sent to the two BSs for transmission to the mobile. However, the transmission paths to the BSs from the BSC can exhibit very different delay characteristics. As used herein, a forward link packet is one generated at a base site controller and transmitted via one or more base sites for reception by a mobile unit. A reverse link packet is one generated and transmitted by a mobile unit for reception at a base site controller via one or more base sites.
Digital cellular systems, including CDMA systems, are subject to relatively long delays. Typically delays from the mobile unit, through the BS, the BSC, the Mobile Switching Center (MSC), the Public Switched Telephone Network (PSTN) and back to the mobile unit can exceed 200 milliseconds (ms). Increased use of packet based networks, the mixing of circuit and packet based networks, and the networking of equipment from different manufacturers will cause delays to increase further. Increased speech delays can negatively impact a user's perception of voice quality when in a conversation. As the roundtrip delay in a cellular system increases beyond 200 ms an increased percentage of users will consider the delay bothersome.
Consequently a need exists for an apparatus and method which facilitates a reduction in system delay while maintaining a high channel throughput.